RFR: 8264873: Dependencies: Split ClassHierarchyWalker

[Vladimir Ivanov]

Split ClassHierarchyWalker into ConcreteMethodFinder and ConcreteSubtypeFinder.

Testing:
- [x] hs-tier1 - hs-tier4

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Depends on: https://git.openjdk.java.net/jdk/pull/3386

Commit messages:
 - Split ClassHierarchyWalker into ConcreteMethodFinder and ConcreteSubtypeFinder

Changes: https://git.openjdk.java.net/jdk/pull/3387/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3387&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264873
  Stats: 438 lines in 1 file changed: 134 ins; 144 del; 160 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3387.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3387/head:pull/3387

PR: https://git.openjdk.java.net/jdk/pull/3387